I. Technical Field
This invention relates to a chain tensioner used to maintain the tension of a timing chain for driving a camshaft of an automotive engine.
II. Description of the Related Art
In an automotive engine, the rotation of the crankshaft is ordinarily transmitted to a camshaft through a timing chain to open and close valves of combustion chambers with the rotation of the camshaft. In order to keep the tension of the chain within an optimum range, a tension adjusting assembly is often used which comprises a chain guide pivotable about a pivot shaft and chain tensioner for pressing the chain guide against the chain.
One known chain tensioner forming such a tension adjusting assembly comprises a cylindrical cylinder having an open end and a bottom, a cylindrical plunger axially slidably inserted in the cylinder and having a bottom and an open end located in the cylinder, a screw rod mounted in the cylinder and having an external thread on an outer periphery thereof, wherein the plunger has an internal thread on an inner periphery thereof which is in threaded engagement with the external thread of the screw rod, and wherein the external thread and the internal thread each have a pressure flank for receiving pressure when a force is applied that tends to push the plunger into the cylinder, and a clearance flank, the pressure flank having a larger flank angle than the clearance flank, whereby the external thread and the internal thread are serration-shaped, and a return spring mounted between the plunger and the screw rod and biasing the plunger in a direction to protrude from the cylinder, thereby pressing the chain with the plunger (JP patent publication 10-19095A).
With this chain tensioner, when the tension of the chain increases while the engine is running, the plunger is pushed into the cylinder under the tension of the chain, thereby absorbing the tension of the chain. At this time, the vibration of the chain allows the screw rod to rotate relative to the plunger while repeatedly moving back and forth within the range of the clearance between the internal and external threads.
When the tension of the chain decreases while the engine is running, the plunger protrudes from the cylinder under the biasing force of the return spring, thereby eliminating slackness of the chain.
When the engine is stopped, the tension of the chain may remain high according to the stop position of the camshaft. In this state, since the chain does not vibrate, the internal thread of the plunger is supported by the external threads of the screw rod, so that the plunger is kept in a stationary position. Thus, when the engine is restarted, the chain is less likely to slacken, thus allowing smooth startup of the engine.